The global body cites an increase in rape and continued attacks by militias and rebel factions.
The UN said on Wednesday that the May 5 peace deal, signed in Nigeria, was "doomed to failure" without more support from the Sudanese government, "with the population of Darfur continuing to suffer grave violations of human rights as violence among competing armed groups in Darfur persists."
The 20-page report by the UN High Commissioner for Human Rights acknowledged that fighting between Sudanese armed forces and the Sudan Liberation Army, the main rebel group, has decreased since the deal.
But "attacks by militias and rebel factions continued unabated, mainly in south and north Darfur", it said.
"Civilian populations continued to be targeted by militia and the government and rebel movements are in breach of the new ceasefire," the report said.
After the signing of the peace agreement, "violence resulted in numerous civilian deaths and aggravated the already severe humanitarian situation in Darfur."
The deal was supposed to help end the conflict, but instead has sparked months of fighting between rival rebel factions.
Aid groups, the UN and beleaguered African Union peacekeepers say rebel factions are seeking to gain advantage before peace upsets a status quo in a region where more than 200,000 people have been killed since 2003, when ethnic African tribes revolted against the Arab-led Khartoum government.
Fresh clashes have left countless dead in the last two months.
The UN reports says armed
militias continue to attack villages
The conflict has caused more than two million to flee their homes, and one million people relying on food aid because their fields were razed or they are too afraid to go out to farm.
The UN report said armed militias continued to attack villages, and on at least one occasion were supported by government forces.
Clashes between militias and the SLA, and between rebel factions, were also cited.
"These attacks also resulted in the torture and killing of civilians and sexual abuse, including rape, as well as in further displacement of the population," the UN said, and noted an increase in rape and attempted rape cases.
Violence also affected humanitarian efforts.
The UN said there were at least 250,000 people who needed aid at the end of June but who could not be reached by organisations because of the insecurity.
Last month, the deadliest month for aid workers since the conflict began, eight Sudanese humanitarian workers were killed in road ambushes when they were working at water pumps, or, in one case, during a night-time village attack.
Aid groups gave warning on Tuesday that conditions for millions of civilians could get much worse quickly if security did not improve in the area.
They said spiralling violence was causing a rise in malnutrition and the spread of disease in some camps for displaced persons.
Aid groups warn that conditions
could get much worse
The UN also said security in Darfur had worsened over the last month, estimating that 25,000 people were newly displaced.
The report called on Sudan's government and all parties of the peace deal to immediately comply with its cease-fire provisions.
"The government should disarm the militia and protect the physical security of all Darfurians by putting in place a credible, capable, and professional police force and judiciary," the report said.
It added that Khartoum needed to do more to investigate reports of sexual violence and bring those responsible to justice - "whether the crime is perpetrated by government agents, armed groups or private individuals."
The UN called on the international community to support the African Union's peacekeeping force in Sudan until a UN operation could be deployed to Darfur.